Optical readout method with a direction of a magnetic field applied to a recording medium changed with a signal, and a system therefor

ABSTRACT

An optical recording medium, optical recording method with use of it, and optical recording device for it comprise at least one magnetic layer on a substrate, the magnetic layer having a temperature Tco at which sizes of domains of the magnetic layer is sharply shrunk or expanded with temperature, thereby being capable of writing and reproducing high-density information formed of smaller domains than an optical spot diameter in a high signal quality.

RELATED APPLICATIONS

This is a continuation of application Ser. No. 08/915,086, filed Aug.20, 1997, now U.S. Pat. No. 5,867,455, which was a file-wrappercontinuation of application Ser. No. 08/350,937, filed Dec. 7, 1994, nowabandoned.

FIELD OF THE INVENTION

The present invention relates to an optical recording medium, opticalreadout method with use of magneto-optical effect, and optical recordingdevice for it that are capable of writing and reproducing high-densityinformation formed of smaller domains than an optical spot diameter in ahigh signal quality.

BACKGROUND OF THE INVENTION

A prior optical readout method with use of magneto-optical effect, asshown in FIG. 10(a) as an example, detects intensity of and polarizationdirection of a reflected light from an optical spot irradiated onto amedium 6 to form a readout signal. This method has the disadvantage thatas shown in FIG. 10(b), the readout signal output extends around twotimes radius ro of the optical spot so that two or more marks within theoptical spot cannot be resolved. The recording density of the priormethod is limited by the diameter (λ/NA) of the optical spot where λ iswavelength of the readout beam and NA is a numerical aperture of afocusing lens 7. If the wavelength is 780 nm, and the numerical apertureis 0.55, for example, the diameter of the optical spot is 1.4 μm, andthe size of the minimum reproducible mark is a half of it, or is limitedto around 0.7 μm.

A prior method of reproducing smaller mark than the diameter of theoptical spot were proposed in U.S. Pat. No. 5,018,119 or the like. Thisprior art, as shown in FIG. 11, uses a magnetooptical recording mediumhaving three or more magnetic layers. A part of the optical spot 2 ismasked to make magnetization of the magnetic layer on the lightirradiation side uniform in a single direction so that it cannot be seenas signal, thereby reproducing smaller domain 1 than the optical spot 2at a high resolution. The masking area is formed at an area of themedium that is heated to higher temperature than a masking temperatureTm by the readout light beam.

As an example, a method of recording smaller mark than the diameter ofthe optical spot is as follows:

(1) Intensity of a light beam to be irradiated is controlled so thatonly the temperature at a center of an optical spot can exceed rightover a recording temperature. This can limit a recording area to ahigh-temperature area (minute area) of the center of the light to formsmaller mark than the optical spot.

(2) The light beam is irradiated to the medium to increase thetemperature of the medium to the recording temperature and at the sametime, a magnetic field is modulated at a high speed to form minutedomains. This method was proposed, for example, in the Japanese PatentLaid-Open No. 54-95240.

The above-mentioned prior art have the disadvantage that the signal areais limited to the part of the optical spot so that the readout output isdecreased. As a result, high S/N cannot be obtained. The above-mentionedprior arts also have the disadvantage that the domain must be formedvery small to record at a high density.

SUMMARY OF THE INVENTION

In view of solving the foregoing problems of the prior arts, it is anobject of the present invention to provide an optical recording medium,an optical readout method, and an optical recording device that arecapable of writing and reproducing information at a high density.

In order to accomplish the above-mentioned object, the present inventionmakes use of an optical recording medium having a magnetic layer havinga characteristic that a size of a domain of the magnetic layer issharply shrunk or expanded at a temperature Tco. That is, the magneticlayer has the characteristic that as shown in FIG. 2, a domain diameterDo at room temperature is sharply shrunk when the temperature isincreased to the temperature Tco, or that as shown in FIG. 5, the domaindiameter of 0 at room temperature is sharply expanded to Do when thetemperature is increased to the temperature Tco, or that a domain ofdiameter Do is suddenly created when the temperature is increased to thetemperature Tco.

In order to stabilize the domain diameter Do to make the size of domainor presence or absence of the above-mentioned shrinking or expandingincident correspond to recording information, the optical recordingmedium to be used should have at least one magneto-optical recordinglayer in addition to the magnetic layer or have the magnetic layerhaving areas therein different in magnetic characteristics in advance asrecorded information.

As an example, a multilayered optical recording medium 6 shown in FIG.19 should be used. This medium has not only a first magnetic layer 51having a domain diameter changed with temperature, but also a thirdmagnetic layer 53 for holding the recording information as presence orabsence of a domain 1 of diameter Do. A second magnetic layer 52 is putin to control a bonding force between the first magnetic layer 51 andthe third magnetic layer 53, but not always needed. To write therecording information in the third magnetic layer 53, we should use thesame method as described in the above BACKGROUND OF THE INVENTION. InFIG. 19, the domain 1 in the third magnetic layer 53 is made tocorrespond to the recording information. Alternatively, it is possibleto use the first layer for the magnetic layer and make a concave andconvex of a substrate 13 or magnetic difference of the magnetic layeritself correspond to the recording information. This method isparticularly effective in a read only memory (ROM).

The optical recording medium of the present invention can accomplishsharp change rate of the readout signal to increase the signal quality(S/N) at a high frequency (high density) in a way that the opticalrecording medium has the readout beam irradiated thereto to locallyincrease the temperature of the magnetic layer to around Tco to changethe domain shape and the change of the polarization state of thereflected light with the shape change is detected to reproduce theinformation. In this case, it is possible to selectively reproduce onlythe portion of the domain shape changing in the optical spot. This meansthat smaller domain than the optical spot diameter can be reproduced ata high resolution.

If time of change of the readout signal can be detected, detectionaccuracy of the change time can be increased. High detection accuracy ofthe instance of the readout signal means that the mark position andlength can be detected more accurately, or that readout can be made fromhigh-density recording.

Also, the optical recording method of the present invention can stablydetect smaller mark than the beam spot with superior high-frequencycharacteristics by detecting the instance of sharp change of the readoutsignal that meets the following relationship:

    ΔT<r.sub.o /v

where ΔT is the change rise or fall time of the readout signal, r_(o) isa radius of the readout beam spot, and v is a relative movement speedbetween the readout beam spot and the optical recording medium (FIGS. 1and 6).

If the change of the polarization state (polarization angle) of thereflected light due to the magneto-optical effects, such as Kerrrotation, is detected as the readout signal, the temperature of themagnetic layer heated by the readout light is increased to Tco when theoptical spot 2 reaches the recording domain 1 of diameter D while theoptical spot moves by ΔT×v in a direction 3 of the optical spot 2 asshown in FIG. 3. Size of the domain 1 then is sharply shrunk orexpanded. This changes the readout signal sharply. The change time isfar shorter than that of the prior recording media. That is, the opticalrecording method of the present invention can change the readout signalat a very high speed, while the prior methods take a time in which theoptical spot of diameter 2ro passes over the domain until the readoutsignal changes over. The change of the readout signal in FIG. 1corresponds to the sharp shrink of the domain, or to the medium of thecharacteristic in FIG. 2, while the change of the readout signal in FIG.6 corresponds to the sharp expansion of the domain, or to the medium ofthe characteristic in FIG. 5.

Further, the optical recording method of the present invention canstably detect more precise mark position or size at higher accuracy bydetecting an instance of sharp change of the readout signal that meetsthe following relationship

    ΔT<D/v

where ΔT is the change time of the readout signal, D is size of therecording marks to be reproduced, and v is a relative movement speedbetween the readout beam spot and the optical recording medium.

The prior art has the disadvantage that as shown in FIG. 11, the domain1 is progressively deleted from an end thereof at the time when thedomain 1 enters an area of T >Tm. For the reason, the signal changetakes a time ΔT' (=D/V) that the optical spot moves over the diameter ofthe recording domain. On the other hand, the optical recording method ofthe present invention has the advantage that as shown in FIG. 3, thesize of the whole domain 1 shrinks sharply when an average temperature Tof the whole domain 1 becomes T>Tco. The domain 1 vanishes suddenly. Thechange ΔT of the readout signal of the present invention, therefore, isshorter and sharper than the prior change ΔT'.

In the method that the optical recording medium has a pulse readout beamirradiated thereto and that a polarization state of a reflected light ofthe readout beam is detected as a readout signal, information canreproduced by detecting an instance of sharp change of the readoutsignal that meets the following relationship:

    ΔT<tp

where ΔT is the change time of the readout signal and tp is length ofpulse of the readout beam. An example of the method is shown in FIGS. 4Aand 4B.

In the method, the laser beam is irradiated to the magnetic layer atcertain intervals like pulse to heat to change the diameter of thedomain 1. As the readout signal is sharply changed only when the domainexists, presence or absence of the recording domain can be detected bydetecting the sharp change. It is therefore possible to detect therecording domain at high speed even if the optical spot does not moverelatively to the medium. The method is suitable to an optical card.That is, as it is difficult to (rotate) the medium of card type at ahigh speed, we use the method that position of the optical spot shouldbe moved to reproduce. In this case, as the optical spot is not alwaysscanned continuously, it is preferable to use the method that theoptical spot should be pulsated to irradiate to the portion from whichinformation is reproduced.

Furthermore, the recorded information can be reproduced in a way thatrecording marks of virtually identical shape are formed at differentdistances as the recorded information and the distances between therecording marks are detected as the sharp change of time of the readoutsignal. The distance between the marks, as shown in FIG. 7, may bemodulated by two or more discrete values, and step width of themodulation may be made narrower than a half of the diameter of therecording mark. Let the distance between the marks be denoted by d. Themodulation is made as

    d=d.sub.o +Δ·n

where d_(o) is the shortest mark interval, Δ is the step width, and n islarger integer than 0. Δ is

    Δ<D/2

where D is the mark diameter.

As described above, the recording marks should be shaped virtuallyidentical and more preferably should be made virtually circle to makethe sharp change of the readout signal occur on all the marks uniformly,thereby being capable of reproducing information stably. Also, the stepwidth of the modulation should be made narrower than the half (radius)of the diameter of the mark to obtain high recording density withoutrecording small marks.

Further, higher recording density can be obtained in a way thatinformation should be recorded on the optical recording medium bychanging both length of the recording marks and distance between therecording marks and by detecting instances of sharp changes of thereadout signal corresponding to a leading edge of any of the recordingmarks and a trailing edge of the recording mark.

Alternatively, information can be recorded and reproduced in a way thata distance between the leading edges of the two adjacent recording marksand another distance between the trailing edges of the recording marksare recorded as different, independent information, the distances aremodulated by two or more discrete values, and a step width of themodulation corresponding to the leading edge is different from the onecorresponding to the trailing edge.

An example is shown in FIG. 12. Change of the readout signal can bedetected accurately without adverse effect of noises as it is sharp wheninstance of the readout signal that crosses over a slice level 4 isdetected. In the example, detection at the trailing edge is made athigher accuracy. Therefore, shorter step width of the modulation at thetime of information recording is more effective to record at highdensity. This makes it possible to set optimum step width depending onchange sharpness (S/N) of the readout signal. It is therefore possibleto make the recording density high at the highest efficiency.

Further, reproduction conditions for the leading edge and the ones forthe trailing edge can be independently optimized in a way that theinformation corresponding to the leading edges can be made to reproduceat a scan time of a laser beam different from the one corresponding tothe trailing edges or that direction of a readout magnetic field appliedto around a light spot for detection of the information corresponding tothe leading edges is changed for detection of the one corresponding tothe trailing edges. This can increase the readout signal quality. Forthe purpose, it is preferable to use a medium having both thecharacteristics shown in FIGS. 2 and 5 for the directions of the readoutmagnetic field.

The optical recording device of the present invention for reproducingthe recorded information from the optical recording medium has anoptical head for irradiating a readout beam to the optical recordingmedium to form a light spot, a polarization detector for detecting apolarization state of a reflected light of the readout beam, a positionservo system for moving the light spot to a desired position of theoptical recording medium, and a selector for selecting only higherportion of a change rate of the readout signal with time than a certainvalue before detecting an instance of the portion. The selector can beformed of high-frequency detector and a comparator.

Such a construction of the optical recording device can accomplish sharpchange rate of the readout signal, thereby increasing the signal quality(S/N). It also can reproduce smaller domain than the optical spot athigh resolution, as it is possible to selectively reproduce only theportion of the domain shape changing in the optical spot.

The optical spot area can have a readout magnetic field applying meansfor applying a readout magnetic field thereto. This makes it easy toreproduce information at high S/N while the reproduction conditions areoptimized.

The optical recording device may further have a controller forcontrolling direction of the readout magnetic field applied to thereadout magnetic field applying means so that the controller can controlthe direction of the readout magnetic field to reverse on the basis ofthe information of instance detected by the selector. This makes itpossible to reproduce information at the edges at high speed while thereproduction conditions for the leading edge and the ones for thetrailing edge can be independently optimized.

The optical recording device may further have at least two sets of theoptical head and the readout magnetic field applying means each. Thismakes it easy to reproduce information while the reproduction conditionsfor the leading edge and the ones for the trailing edge can beindependently optimized. The device can make readout more stably.

As described so far, the present invention makes deformation(extinction, generation, or expansion) of the domain on the opticalrecording medium to cause sharp change of the area of the domain in theoptical spot. The sharp change is reflected to the readout signal.Therefore, the optical recording method of the present invention canmake the readout signal have the portions changing very sharply. Thesharpest portion of the readout signal can be used effectively toaccomplish the high-density recording and reproduction.

As the readout signal is sharp, possible superimposition of amplitudenoises on the readout signal can be made less in effect on jitter(small, rapid fluctuation) of the readout signal. Therefore, thedetection accuracy of the instance of change of the readout signal canbe increased. The high detection accuracy of the instance of change ofthe readout signal means that the mark position and length on therecording medium can be detected accurately. That is, it is possible toreproduce information at high density.

To cause the area change of the domain, we should use a medium having acharacteristic of sharp change of the recording domain diameter withtemperature as shown in FIGS. 2 and 5. Such a medium can be obtained,for example, by making the magnetic layer multiple as shown in FIG. 19.The medium having the characteristic can achieve the sharp change of thereadout signal as shown in FIGS. 1 and 6.

To detect the sharp change of the readout signal efficiently, we can usecombination means of a differential circuit and slicing circuit fordetecting only high-frequency components. The combination means canprevent detection of signal components that do not change sharply. Thisis effective to eliminate inter-track crosstalk and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a graph illustrating an embodiment of the optical readoutmethod according to the present invention;

FIG. 2 depicts a characteristic graph illustrating an example of theoptical recording medium according to the present invention;

FIG. 3 depicts a sketch and graph illustrating an embodiment of theoptical readout method according to the present invention;

FIGS. 4A and 4B depict graphs illustrating an embodiment of the opticalreadout method according to the present invention;

FIG. 5 depicts a characteristic graph illustrating an example of theoptical recording medium according to the present invention;

FIG. 6 depicts a graph illustrating an embodiment of the optical readoutmethod according to the present invention;

FIG. 7 depicts a sketch illustrating an embodiment of the opticalreadout method according to the present invention;

FIGS. 8A and 8B depict graphs illustrating an embodiment of the opticalreadout method according to the present invention;

FIGS. 9A and 9B depict graphs illustrating an embodiment of the opticalreadout method according to the present invention;

FIGS. 10(a) and 10(b) depict a simplified sketch and graphs illustratinga prior optical readout method;

FIG. 11 depicts a sketch and a graph illustrating an example of theprior optical readout method;

FIG. 12 depicts a graph illustrating an embodiment of the opticalreadout method according to the present invention;

FIG. 13 depicts a block diagram illustrating an embodiment of theoptical recording device according to the present invention;

FIG. 14 depicts a sketch illustrating an embodiment of the opticalrecording device according to the present invention;

FIG. 15 depicts a graph illustrating an embodiment of the opticalreadout method according to the present invention;

FIGS. 16A and 16B depict graphs illustrating principles of the opticalreadout method according to the present invention;

FIG. 17 depicts a graph illustrating principles of the optical readoutmethod according to the present invention;

FIGS. 18A, 18B and 18C depict graphs of the clock signal, the readoutsignal and the reproduced signal, respectively, of an embodiment of theoptical readout method according to the present invention; and

FIG. 19 depicts a partially cross-sectioned view illustrating anembodiment of the optical recording medium according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1: Optical Recording Medium

FIG. 19 depicts a cross-sectioned view illustrating an embodiment of theoptical recording medium 6 according to the present invention. Adisk-like substrate 13 having grooves of 0.8 μm intervals has a SiNdielectric layer of 60 nm thick laminated thereon. The substrate 13further has a first magnetic layer 51 of Gd₂₃ Fe₅₉ Co₁₈ of 40 nm thickhaving a low coercivity, second magnetic layer 52 of Tb₁₈ Dy₁₀ Fe₇₂ of10 nm thick, and third magnetic layer 53 of Tb₂₈ Fe₆₁ Co₁₁ of 50 nmthick laminated in the sequence thereon to form a magnetic layer 50 byway of an rf magnetron sputtering method. Total thickness of themagnetic layer 50 therefore is 100 nm. The magnetic layer 50 further mayhave a dielectric protection layer, such as SiN, formed thereon asnecessary.

The optical recording medium 6 formed as described above was magnetizedby applying stronger magnetic field than 25 KOe thereto. After this, theoptical recording medium 6 was made to have domains of diameter D_(o) of0.35 μm to write in a way that it was subjected to irradiation ofintense laser pulse of 15 mW and 20 nsec while having a bias magneticfield of 400 Oe applied thereto. After writing, as shown on a left sideof FIG. 19, the domain of diameter Do was formed between the firstmagnetic layer 51 and the third magnetic layer 53.

The second magnetic layer 52 of the recording medium has as low a Curietemperature as 120° C. If the medium was heated to around thetemperature while having a readout magnetic field Hr applied thereto,the magnetization of the second magnetic layer 52 was eliminated, andmagnetizations of the first magnetic layer 51 and third magnetic layer53 were decoupled magnetically. For the reason, as shown in FIG. 2, thedomain diameter D of the first magnetic layer 51 having low coercivitywas made small so quickly that the domain was collapsed. The collapsingtemperature Tco was 105° C.

The second magnetic layer 52 is effective to control an exchange forcebetween the first magnetic layer 51 and the third magnetic layer 53.But, the second magnetic layer 52 is not always needed.

Embodiment 2: Optical Recording Medium

The following describes in detail a second embodiment of the opticalrecording medium 6 for use in the present invention. The opticalrecording medium 6 was formed as described below.

A disk-like substrate 13 having concave pits of 0.25 μm diameter and 10nm deep has a SiAlON dielectric layer of 50 nm thick laminated thereon.The substrate 13 further has a first magnetic layer 51 of Gd₁₃ Dy₈ Fe₆₁Co₁₈ of 25 nm thick having a low coercivity, a second dielectric layerof SiN of 15 nm thick, and a reflection layer of Al-Ti alloy of 60 nmthick laminated in the sequence thereon by way of an rf magnetronsputtering method. The substrate 13 further may have an organicprotection layer coated thereon as necessary.

The optical recording medium 6 formed as described above was magnetizedby applying stronger magnetic field than 25 KOe thereto to form domainsof 0.25 μm diameter at the concave pits. In general, the magnetronsputtering method makes thin the layer thickness of the concave,changing the magnetic characteristic. In the second embodiment, as aresult of the coercivity that was made slightly lower, a demagnetizingfield from a periphery of each pit forms the domain automatically. Thedisk had a weak bias magnetic field of 150 Oe applied thereto in thesame direction as the first embodiment and was heated while beingobserved through a microscope. The result of observation of the domaindiameter at the pit is shown in FIG. 2. A collapsing temperature Tco atwhich the domain was collapsed was 125° C.

Embodiment 3: Optical Recording Medium

The following describes in detail a third embodiment of the opticalrecording medium 6 for use in the present invention. The opticalrecording medium 6 was formed as described below.

A disk-like substrate 13 having grooves has a SiON dielectric layer of55 nm thick laminated thereon. The substrate 13 further has a firstmagnetic layer 51 of Tb₂₂ Fe₇₂ Co₆ of 25 nm thick, a second dielectriclayer of SiN of 15 nm thick, and a reflection layer of Al-Ag alloy of 40nm thick laminated in the sequence thereon by way of an rf magnetronsputtering method. The substrate 13 further may have an organicprotection layer coated thereon as necessary.

The optical recording medium 6 formed as described above was changed inits magnetic characteristic at an area of 0.25 μm diameter to writeinformation by use of a laser beam of 351 nm wavelength as strong as 15mW. That is, the coercivity of the medium was made to decrease. It isthought that such a phenomenon that the coercivity was made to decreaseby the strong laser beam is due to the fact that an amorphous state ofthe medium is relaxed structurally. The disk was magnetized by applyingstronger magnetic field than 25 KOe thereto and had a bias magneticfield of 400 Oe applied thereto in an opposite direction. The disk washeated while domain diameters at pits thereof were observed through amicroscope. The result of observation of the domain diameter is shown inFIG. 5. A temperature Tco at which the domain was generated was 140° C.

The third embodiment described is a ROM-type medium that writesinformation in advance by way of strong laser beam. The embodiment, ofcourse, can be used as DRAW-type.

Embodiment 4: Optical Readout Method

The following describes in detail a fourth embodiment in which theoptical readout method of the present invention is used in the opticalrecording medium 6 described in the first embodiment by reference toFIG. 3.

Left views in the figure are looked from the first magnetic layer 51 ofthe optical recording medium 6. In the upper left view, suppose asituation that an optical spot 2 was relatively moved into an areahaving a domain I formed therein in a direction of an arrow 3. Theoptical spot 2, as shown in FIG. 19, was formed by focusing the laserbeam through a lens 7.

Radius of the optical spot 2 was 0.6 μm as a wavelength thereof used was680 nm and the numerical aperture of the lens used was 0.55. The laserbeam used was DC beam of 3 mW. The optical spot 2 was moved at arelative speed v of 1.2 m/sec to the medium. This forms behind theoptical spot 2 in FIG. 3 an area having higher temperature T than thecollapse temperature Tco. The area moves together with the optical spot2.

In the upper left state in FIG. 3, as the recorded domain 1 graduallycomes to a center of the optical spot 2, the readout signal increasesgradually. When the optical spot 2 comes to a position shown in thelower left view of the figure, the domain 1 is heated right to thecollapse temperature Tco. With this, the recorded domain diameter D, asshown in the figure, becomes narrow sharply. At the same time as thedomain collapses, the readout signal decreases abruptly (FIGS. 16A and16B). Note that sign of the readout signal can be reversed from the onesin FIGS. 3 and 16A, 16B depending on the polarity of an amplifier of theoptical recording device of the present invention.

For the reason, the change of the readout signal can be made sharperthan the prior one having no collapse. In the fourth embodiment, thechange time (rise time or trailing time) ΔT of the readout signal was 80nsec. The movement of ΔT×v=0.096 μm shown in FIG. 1 is far shorter thanthe radius ro of the optical spot 2 and far shorter than the recordeddomain of 0.35 μm. In other words, the readout signal in the fourthembodiment is made sharper than the prior ones shown in FIGS. 10 and 11.

In turn, the following describes principles in which the sharp readoutsignal allows the position of the domain 1 to be detected at higherprecision. Assume that as shown in FIG. 17, the readout signal ischanged up and down by shot noises or the like. If the readout signal isnot made sharper, jitter increases as shown at a leading edge of thenoise in the figure. If the readout signal is made sharper, on the otherhand, there occurs very little jitter as at a trailing edge of the noisein the figure. This means that it is possible to correctly detect aninstance when the readout signal changes sharply. The instance of thesharp change of the readout signal is an instance when the recordeddomain reaches right to the collapse temperature Tco. The devicetherefore can accurately detect presence or absence of the recordeddomain at the position of the collapse temperature Tco in the opticalspot.

On the contrary, by detecting only the sharp change of the readoutsignal, the device can detect only the information at the position ofthe collapse temperature Tco in the optical spot. The device thereforecan detect smaller domain than the optical spot without interference. Ifthe groove intervals are made narrower, of course, the device also cansuppress occurrence of the crosstalk that the information of adjacenttracks mixes into the current one.

Amplitude of the noise in the fourth embodiment was 1/4 of that of thereadout signal; that is, SN of the readout signal was 12 dB. In thatcase, the jitter was around 1/4 of ΔT; that is, the position of therecorded domain can be detected at an accuracy of 0.024 μm.

With the use of the performance that the position of the recorded domaincan be detected accurately as described above, a high-density recordingcan be made if information is written in such a way as in FIG. 7. Thatis, let the intervals of the domain 1 be denoted by d. Then, d=do+Δ·nwhere do is a shortest mark interval, Δ is a step width, and n is 0, 1,2, 3, and so on. As an example, a VFM modulation having do=0.6 μm wasused. In the modulation, Δ0.12 μm, n=0 to 18, and the bit density is 0.3μm/bit. As the groove interval is 0.7 μm, the recording density is 3Gb/in².

Embodiment 5: Optical Readout Method

FIG. 15 depicts a fifth embodiment in which the present invention isused in a mark edge recording. In the fifth embodiment was used theoptical recording medium described in the first embodiment. For recodinginformation, the magnetic field modulation was used. This method ofmodulation is useful for the mark edge recording as it can write minutedomains with a laser beam of relatively long wavelength. In theembodiment was used the NRZ edge recording with the laser beam of 680 nmwavelength, 0.5 μm width, and 0.3 μm shortest mark interval.

Readout of recorded information was made while a bias magnetic field wasmodulated in a real time way. That is, for detecting the leading edge,the bias magnetic field was modulated upward to read out. Right afterdetection of the leading edge, the bias magnetic field was reversed forpreparation of detection of the trailing edge. As the readout magneticfield was changed at the times of detections of the leading and trailingedges, the device can obtain such a readout signal waveform havingsharpness both at the leading and trailing edges as shown in FIG. 15. Asthe fifth embodiment uses the magnetic field modulation for recording,it can make overwriting easily.

An alternative example of mark edge detection is a method shown in FIG.14. In the method is used a light spot 21 for leading edge detection anda light spot 22 for trailing edge detection. For the light spots areused two sets of optical heads and readout magnetic field applying means8 to read out the leading and trailing edges. Another alternativeexample of mark edge detection is a method shown in FIGS. 9A and 9B. Inthe method, a trailing edge is read out at a downward bias magneticfield in a first rotation, and a leading edge is read out at an upwardbias magnetic field in a second rotation. In either of the methods, themark edge recording can make high-density recording about 1.5 times ascompared with the mark position recording in the second embodiment. Thatis, the mark edge recording can easily accomplish the higher densityrecording than 4 Gb/in². The mark edge recording, however, needs moreprecise write control of the domain length.

Embodiment 6: Optical Recording Device

FIG. 13 depicts a block diagram illustrating an optical recording devicewhich is a sixth embodiment according to the present invention.

The optical recording device uses the optical recording medium 6described in the first embodiment and has an optical head that canirradiate light to write on or read out of the optical recording medium6. The optical head is positioned to an information writing or readoutpoint by an automatic position servo system, including autofocusing andtracking means. To write the information, a modulator is used tomodulate a light intensity and a magnetic field intensity depending onthe information to be written. The optical recording device furtherincludes a readout magnetic field applying means 8 and a readoutmagnetic field 9.

For readout, the light and the magnetic field are applied while thelight intensity and the magnetic field intensity are controlled forquality readout. The optical recording device has a readout system,including a polarization detector, for detecting a polarization plane ofa reflected light. The readout signal is led to a selector thatdifferentiates the signal before slicing it so that only higher portionof the differentiative signal can be detected.

As shown in FIGS. 8A and 8B, the differentiated signal of the readoutsignal is sliced at a slice level 4. A sharp change AT of the readoutsignal can be selectively detected. Controlling the above-mentionedlight intensity and magnetic field intensity can be made so simply thatamplitude of the differentiated signal should be maximum.

In actual information reproduction, a clock signal 10 shown in FIGS. 18Ais generated on the basis of the readout signal. A reproduced signal 12shown in FIG. 18C is obtained by making `1` and `0` correspond towhether or not the sharp change of the readout signal 11 shown in FIG.18B is detected within a single period of the clock signal.

It will be understood that the present invention is not limited to thespecific embodiments hereinbefore discussed. For example, the firstmagnetic layer 51 may be made of multi-layer of Pt and Co as therecording medium. The layers may be replaced by rare earthtransition-metal alloys of other compositions, including GdFeCo, TbFeCo,DyFe, DyFeCo, TbDyFeCo, GdTbFeCo, NdFeCo, NdDyFeCo, NdTbFeCo, and GdCo.The recording medium may have an additional reflection layer or thermaldiffusion layer. The recording medium may be shaped like a card.

As described so far, the present invention can accurately detect therecorded mark position without adverse effect by shot noise as thereadout signal can be sliced because of the sharp change of the readoutsignal with time. The cross-talk from the adjacent tracks can becompletely eliminated in the way that only the high-amplitude portion ofthe differentiated signal is selectively detected as the amplitude ofthe differentiated signal can be made high. At the same time, very smallmark can be reproduced stably without interference. For the reason, thepresent invention can easily accomplish higher density recording than 3Gb/in².

What is claimed is:
 1. An optical readout method comprising the stepsof:providing a magneto-optical recording medium, applying a magneticfield to said magneto-optical recording medium, irradiating saidmagneto-optical recording medium with a readout beam; reproducing asignal from a portion of said readout beam from said magneto-opticalrecording medium, and changing a direction of said magnetic field insynchronism with said signal.
 2. An optical readout method according toclaim 1, wherein said direction of said magnetic field is changed suchthat said direction of said magnetic field at a time of detectingleading edges of marks recorded on said magneto-optical recording mediumis different from said direction of said magnetic field at a time ofdetecting trailing edges of said marks.
 3. An optical readout methodaccording to claim 1, wherein said direction of said magnetic field isreversed immediately after detecting leading edges of marks recorded onsaid magneto-optical recording medium.
 4. An optical readout systemcomprising:an optical head for irradiating a readout beam to amagneto-optical recording medium, a signal reproducing section forreproducing a signal from a reflected light of the readout beam from themagneto-optical recording medium, a magnetic field applying section forapplying a magnetic field to the magneto-optical recording medium, and amagnetic field controlling section for changing a direction of themagnetic field in synchronism with the signal.
 5. An optical readoutsystem according to claim 4, whereinsaid magnetic field controllingsection makes the direction of the magnetic field at a time of detectingleading edges of marks recorded on the magneto-optical recording mediumdifferent from the direction of the magnetic field at a time ofdetecting trailing edges of said marks based upon the signal.
 6. Anoptical readout system according to claim 4, whereinsaid magnetic fieldcontrolling section reverses the direction of the magnetic fieldimmediately after detecting leading edges of marks recorded on themagneto-optical recording medium based upon the signal.
 7. A method ofreproducing information which has been previously recorded on amagneto-optical recording medium, said method comprising:applying amagnetic field having a bias in a first direction to a recording markstoring said previously recorded information; irradiating a leading edgeof said recording mark with a readout beam while said magnetic fieldhaving a bias in a first direction is applied to the recording mark, tothereby detect a leading edge of said recording mark; reversing apolarity of said magnetic field to apply a bias in a second direction tosaid recording mark, after said leading edge has been detected; andirradiating a trailing edge of said recording mark with a readout beamwhile said magnetic field having a bias in a second direction is appliedto the recording mark, to thereby detect a trailing edge of saidrecording mark.